Charging advice for a weekend-only car

[Tofuprince]

Hi Guys,

I am getting a MYP delivered next week and wanted your advice on the charger and battery percentage. I work from home, so driving the car on weekdays is unnecessary. I will use the car on weekends to take the family out and probably use about 50-60 miles per day on Saturdays and Sundays.

I plan to charge the car in the garage, so the cost-effective option seems to be installing a NEMA 14-50 plug and using the mobile charger. We got a quote to install the NEMA 14-50 at $350 + the charger for $230 = $580. The quote for installing the TWC is $425 + the charger for $425 = $850.

Here are my questions.

• Other than charging speed, are there any differences in using the TWC vs the mobile connector?
• What state of charge should I set the MYP to if I only drive on the weekends? (Battery degeneration is my biggest concern as I plan to keep the car for a while.)
• We sometimes get thunderstorms here in Dallas, do I really need to unplug the car even with the 50amp breaker?

Thank you for the help in advance!

 

[Big Earl]

Mobile Connector vs Wall Connector has been discussed at length. My personal preference is for the Wall Connector due to fewer points of failure (hard wired vs plugged in). You may prefer to use the Mobile Connector in order to have a portable device for trips, or to have a 50 amp receptacle available for other purposes like a welder or an RV.

As for charging, many here will recommend that you keep the car plugged in all the time. I don’t really subscribe to that philosophy, and in your situation, would probably plug in on Friday night, charge to 80% or 90% for the weekend, then let it sit all week, unplugged, at around 40% state of charge. This will keep your average state of charge around 50% and you should have excellent long term battery life.

Prepare for a bunch of different opinions. At the end of the day, it doesn’t make that much of a difference, so do what works best for your situation.

 

[Rocky_H]

I plan to charge the car in the garage, so the cost-effective option seems to be installing a NEMA 14-50 plug and using the mobile charger. We got a quote to install the NEMA 14-50 at $350 + the charger for $230 = $580. The quote for installing the TWC is $425 + the charger for $425 = $850.

As @Big Earl mentioned, this has been discussed a lot, and something is wrong with those quotes in the detailed breakdown. A wall connector installation all-in should not be more than an outlet installation for the same amp level of circuit. It should be just slightly cheaper or about the same.

 

[gaswalla]

based on your needs - keep car charged to 60 percent to maintain battery health, and have enough range (based on your stated needs), keep car plugged in (per Teslas stated recommendation), and dont stress about the 240 install. You actually will be fine w plugging the mobile charger to a 120v standard plug. You’ll get 4 miles back per hour of charging, which is good enough for your planned usage. That being said, I’d encourage you to eventually get a 240 V plug or wall charger to give you that great flexibility of charging fairly quickly at home.

 

[Pianewman]

Sorry, I just couldn't get past the second sentence w/o snickering.

Good luck only driving your new Tesla MYP on the weekends. HAHAHA! (I live in Fort Worth, currently getting hammered by a thunderstorm, so I agree with BigEarl's advice to leave it unplugged.)

 

[brkaus]

I’m not sure why the WC would be more for the install part, if installed at the same max amp level. The 14-50 requires the outlet and a gfci breaker to be purchased if installing up to code.

 

[jcanoe]

I would leave the Tesla Model Y unplugged when thunderstorms are in the weather forecast. The thing about ground strike lightning is that it can travel a good distance (hundreds or even thousands of feet) horizontally once touching earth and enter your home through the electrical wiring and even the house ground connection.

The chance of lightning striking close to your home and entering your home's wiring is low. The potential damage to your Tesla Model Y when plugged in is high enough that you should take precautions whenever storms are active and unplug whenever possible.

 

[zman19]

As @Big Earl mentioned, this has been discussed a lot, and something is wrong with those quotes in the detailed breakdown. A wall connector installation all-in should not be more than an outlet installation for the same amp level of circuit. It should be just slightly cheaper or about the same.

Could be 6/3 vs 4/3 or something?

 

[Pianewman]

I would leave the Tesla Model Y unplugged when thunderstorms are in the weather forecast. The thing about ground strike lightning is that it can travel a good distance (hundreds or even thousands of feet) horizontally once touching earth and enter your home through the electrical wiring and even the house ground connection.

The chance of lightning striking close to your home and entering your home's wiring is low. The potential damage to your Tesla Model Y when plugged in is high enough that you should take precautions whenever storms are active and unplug whenever possible.

...that's why, in the Texas DFW region, I recommend leaving the car unplugged if unattended.

 

[Rocky_H]

Could be 6/3 vs 4/3 or something?

Yeah, that might be. If it's a decently long run and one is using thicker wire, that could make some of that cost difference. With the newer requirement of plug-in receptacles for charging requiring finnicky and expensive GFCI breakers, it's just not worth doing that any more. Even if it were only a 30 or 40A circuit, I would still recommend just doing a wall connector on it because of that damn breaker and the extra weak connections that a plug-in situation has. The wall connector doesn't have to be on a 60A circuit. You configure it for what sized circuit you're putting it on.

And the other wire difference is that if you are doing a 14-50, that does require four wires, because it is 120V/240V both, so needs to have a neutral to be to code. The wall connector is 240V only, so it doesn't need the neutral wire at all. So this should actually be some savings on the wire, not needing that extra one for the wall connector.

 

[herrlich98]

Heck for weekend only driving you might be able to just use a 110V outlet. Only thing that wouldn't work for is if you wanted to do two long trips on both Saturday and Sunday without visiting a Supercharger Sat pm/Sun am.

Not sure what to say about leaving it plugged in all the time with lightning risk... the manual will tell you to Always Be Charging and 100% for LPF batteries and 80-90% for other batteries. I have a strong tendency for convenience rather than signing up for tasks that require more "care and feeding". You can put a surge protector or UPS in a 110V circuit or tell the electrician who installs a 220V circuit you are extra concerned and maybe they can add extra grounding. Your home owners insurance will very likely cover lightning damage to your home electronics and plugged in EV car.

 

[Pianewman]

Heck for weekend only driving you might be able to just use a 110V outlet. Only thing that wouldn't work for is if you wanted to do two long trips on both Saturday and Sunday without visiting a Supercharger Sat pm/Sun am.

Not sure what to say about leaving it plugged in all the time with lightning risk... the manual will tell you to Always Be Charging and 100% for LPF batteries and 80-90% for other batteries. I have a strong tendency for convenience rather than signing up for tasks that require more "care and feeding". You can put a surge protector or UPS in a 110V circuit or tell the electrician who installs a 220V circuit you are extra concerned and maybe they can add extra grounding. Your home owners insurance will very likely cover lightning damage to your home electronics and plugged in EV car.

I think jcanoe explained the variables very clearly. You're in NH, so, AFAIK, you're not prone to severe lightning/tornado activity, etc.

Since I live in "tornado alley," with frequent electrical storms, I'll keep mine unplugged. No need to challenge my house AND my Tesla with potential damage and insurance claims. (Daughter's Prius got hammered with hail yesterday...shee-itt!!!!)

 

[dafish]

If Sentry off, and truly not used, it doesn't hurt to be unplugged during the work week, but doesn't help much either. Lightning I suppose.

There is plenty of evidence that supports the "Charge often, or always, to the lowest amount you need to get to the next recharge opportunity" school of thought. Enough that I've no doubt it's correct. While charging above 70% is known to have mild implications, it's reputed to be relatively small until over 80%.

Our standard daily "need" is low, but screw that, I want to be able to impulse take the thing across town, to next town, whatever. And I STILL want room for an emergency "oh shite" moment. As a result I keep mine set to charge to 50% (at least 100 miles in any weather) and plug in every time it comes home. For known trips or days that might push that I bring it up to 63% in the morning using "Scheduled departure". Then back to "charge to 50% on arrival home". Easy, fully automated, nobody would argue I wasn't well within best practice. I charge on a 24ano 240v circuit, so I recover OK, and that helps keep the 50% level effective.

Saturday it has to go 120 to anotherr state, putx around for 40, and back 120. I'm not making it on one charge, so I'm not going to try. I'll charge to 85% before launch and destination charge back up to 75% for the return, then back to the "on arrival home recharge to 50%" automated (Tessie) routine it goes.

The electrician is likely charging more the "Commissioning" of the WC. I would explicitly ask the electrician what wire gauge and type he was going to use and I'd report that back here. Way too many "it'll be fine, that's the way I always do it" stories here. Tell us FIRST.

Was I having an electrician do this there is no way I would not install a 60 amp circuit and a WC. The added wire cost should from 50 amp should be incremental. If major just put in the 50. Plenty fast at 40a charge rate. Mind you I keep a mobile charger in the car too, so it's not like I don't respect them.

 

[gsmith123]

probably use about 50-60 miles per day on Saturdays and Sundays.

As a couple people have pointed out if you really only drive it that much, 120V 12A charging is more than enough. I could never drive a fun Tesla that little though (lol).

Another option to consider is a NEMA 5-20 outlet - a 120V 20A (16A continuous) outlet, especially if you happen to have one in the garage. You can plug the mobile connector in with an adapter in get about 50% faster charging speeds than 120V 12A charging (it's 50% faster thanks to fixed overhead to run various computers and electronics during charging, so more of the energy goes to the battery the faster you charge).

You'll get 2-2.5% SOC per hour.

 

[dafish]

Many, it not all, "Charge to a lower SoC" advocates don't recommend 120V. The recovery time, and/or "oh shite" time is just too high.

 

[gsmith123]

That's true, if you're just using 120V leave the charge limit at 80-90%

 

[herrlich98]

Another 120V minor downside is that 120V charging is less efficient and more expensive than 220V charging. I don't think the difference is huge. In other words, ignoring that it will take longer if you charge from 20% up to 80% using 120V you will get charged more on your electric power bill than using 220V. There are "losses" getting the energy from the outlet into your car and those "losses" are greater using 120V than 220V. Not sure how long it would take to recover that minor extra cost vs installing a 220V charger but it is many years.

 

[STS-134]

• Other than charging speed, are there any differences in using the TWC vs the mobile connector?

Apparently the WC can be connected to WiFi (which I already knew) and you can gather statistics from it (which I didn't know). Wall Monitor for Tesla | Wall Monitor

• What state of charge should I set the MYP to if I only drive on the weekends? (Battery degeneration is my biggest concern as I plan to keep the car for a while.)

If battery degradation is your biggest concern, then charge to 50%. Then if you need more for a specific trip, set the charge level you need, and set Scheduled Departure for about an hour before you expect to leave.

• We sometimes get thunderstorms here in Dallas, do I really need to unplug the car even with the 50amp breaker?

What does the breaker size have to do with lightning protection? If you want lightning protection, put a whole house surge suppressor on your main panel or subpanel.

 

[STS-134]

Another 120V minor downside is that 120V charging is less efficient and more expensive than 220V charging. I don't think the difference is huge. In other words, ignoring that it will take longer if you charge from 20% up to 80% using 120V you will get charged more on your electric power bill than using 220V. There are "losses" getting the energy from the outlet into your car and those "losses" are greater using 120V than 220V. Not sure how long it would take to recover that minor extra cost vs installing a 220V charger but it is many years.

It's actually much worse than the resistive losses in the wiring. It's also the fact that the car's computer runs continuously (at 150-300W) whenever the car is charging and thus, the overhead sapped by the computer increases as power level decreases. At 120V/12A (NEMA 5-15), this overhead is about 10-20%. At 240V/48A, this overhead is only about 1-3%. It's better to charge as fast as possible (but all AC charging is still "slow" for the purposes of battery degradation and lifespan) and then let the car sleep. I calculated the payback time for installing a 240V circuit for a friend whose aunt bought a Tesla and assumed about 500 miles/month of driving and it was on the order of 1.5 years, mostly because of the cost of running the computer during charging.

 

[gsmith123]

TLDR if you drive little enough that 120V charging is practical, then 240V charging isn't worth it financially (for the efficiency gains).

Assuming 120V charging is 20% less efficient (I think that number is high, but not sure) and electricity costs about 10 cents per kilowatt hours, then you're losing 2 cents per kilowatt hour.

If the 240V charging install costs $500 it'll take 20,000 kWh to break even, or about 60,000 miles. So not entirely unreasonable to invest in 240V charging purely on financial grounds if you drive a lot - however, if OP only drives 100 miles per weekend they the payoff period would be over 10 years.

(plus if you do drive a lot you probably need faster charging anyway)